1. Field of the Invention
The invention relates to integrated circuits, and in particular, to a compact, stable, and accurate reference voltage generator.
2. Related Art
A voltage reference circuit in an integrated circuit (IC) provides a reference voltage for use by other elements in the IC. A bandgap reference generator, which is the most commonly used type of voltage generator, bases its reference voltage on the bandgap voltage of the semiconductor material on which the bandgap reference generator is formed. For example, a bandgap reference generator formed on silicon will produce a reference voltage roughly equal to the bandgap of silicon (1.21 V).
A bandgap reference generator generates a reference voltage Vref by combining the base-emitter voltage Vbe of a bipolar transistor with a proportional-to-absolute-temperature (PTAT) voltage Vt (i.e., Vt increases as temperature increases). Meanwhile, the base-emitter voltage Vbe has an inverse linear relationship with temperature (i.e., Vbe decreases as temperature increases). Therefore, by properly combining base-emitter voltage Vbe with PTAT voltage Vt, it is possible for a bandgap reference generator to produce a reference voltage Vref that is relatively stable across a wide range of temperatures.
Unfortunately, due to manufacturing variations and limitations, the actual output of a bandgap reference generator is often inaccurate (i.e., not at the desired reference voltage level) and/or unstable (e.g., excessively temperature-dependent). Compensation and correction circuitry for that non-ideal output can increase the area, complexity, and cost of ICs that include such bandgap reference generators.
Furthermore, because the reference voltage produced by a bandgap reference generator is fundamentally based on the bandgap of the underlying semiconductor material, the actual reference voltage produced by a bandgap reference generator cannot be easily set to a desired value. Once again, additional voltage adjustment circuitry is required to create a desired voltage from the reference voltage provided by the bandgap reference generator.
Accordingly, it is desirable to provide a simple, stable reference voltage generator that can be flexibly configured to provide a desired reference voltage.